This study was carried out to obtain quantitative SPECT images by correcting attenuation and scattering of gamma rays.In the attenuation correction, we used Chang's method with an accurate map of linear attenuation coefficients measured by gamma ray fanbeam transmission CT (FTCT).FTCT scanning was performed using a commercially available gamma camera system (Toshiba GCA-9300A) with fanbeam collimators.As an external source for FTCT,we used a Tc-99m line source positioned at the focal point of each fanbeam collimator.Scatter correction was performed by eliminating the counts of scattered photons from each planar image.The method used was the triple energy window (TEW) method, in which the count data were acquired at each pixel using three windows : amain window centered at photopeak energy and two subwindows on both sides of the main window.The scattered photons included in the main window were estimated as a trapezoidal region shaped by the counts of the subwindows and the width of the main window, and were subtracted from the count of the main window.These attenuation and scatter correction methods were evaluated by simulations using a Monte Carlo method and by experiments including phantom studies and preliminary clinical studies.The results of these simulations and experiments showed that quantitative measurement of activity in an object was realized in clinical situations.